The Pathfinder Martian probe has been one small step for NASA and the Jet Propulsion Laboratory, and one giant leap for...Mattel? Sudden interest in the fourth rock from the sun has sent demand for the Hot Wheels version of the Sojourner exploration vehicle into orbit. When the Mars Rover Action Pack shipped to stores in May, initial sales of the $5 toy were less than spectacular. (Mattel declines to release specific figures.) But with Mars mania in full swing, toy stores nationwide are selling out of the mini-space probes. The gift shop at JPL headquarters in Pasadena unloaded its stock of 1,500 models in less than 20 minutes. ''They were selling them right out of the shipping cartons,'' says Sara Rosales, Mattel's manager of public relations. ''We had no idea that this would get so large.'' Which explains why toy Rovers are now as scarce as water on the red planet, though Mattel promises to have them back on store shelves within a week or so. Project engineers at JPL, however, won't be waiting on back orders. Mattel sent each of the Sojourner's designers a Hot Wheels of his or her very own.